Currently, a passive optical network (Passive Optical Network, PON) has gradually become a mainstream technology in a broadband access field. With rapid development of various broadband services, for example, video conference, three-dimensional (Three-Dimensional, 3D) television, mobile backhaul, and interactive game, higher requirements are imposed on an access bandwidth. On an optical communication network, especially a PON system with a very tight power budget, sensitivity of a receiver plays a critical role. In a coherent reception technology, a local-oscillator light with relatively high power is introduced to amplify a received signal, and meanwhile a optoelectronic receiver works in a shot noise-dominant state, so that a shot noise limit of the receiver can be reached, thereby greatly increasing the sensitivity and spectral efficiency of the receiver.
In the prior art, a central office device, optical line terminal (Optical Line Terminate, OLT), generates ultra-dense wavelength division multiplexing (UD-WDM) signals with a C-band wavelength spacing of 3 GHz, which are received by each optical network unit (Optical Network Unit, ONU) through an optical distribution network (Optical Distribution Network, ODN) based on an optical splitter (splitter); a tunable laser is set on each ONU to generate a local-oscillator optical signal; by means of adjusting a wavelength of the local-oscillator optical signal to a position that has a 1 GHz difference from a target wavelength, one signal may be selected randomly from the received UWDM signals to perform coherent reception. However, in the prior art, the ONU requires a high-cost tunable laser to generate a local-oscillator optical signal, and needs to use a complex polarization diversity receiving structure to perform coherent reception on the local oscillator optical signal and a received downlink signal.